This invention relates to a particulate trap for collecting and removing particulates such as carbon contained in exhaust gas discharged from a diesel engine.
Exhaust gas discharged from cars is a major cause of air pollution. It is very important to develop techniques for removing harmful components contained in the exhaust gas. It is especially important to develop techniques for removing particulates in the exhaust gas discharged from diesel engines, which contain .DELTA.NOx and carbon.
Heretofore, various efforts have been made to remove such harmful components from the exhaust gas. Such efforts include putting EGR (exhaust gas recirculation) on the engine and improving the fuel injection system. However, none of these efforts has been a decisive solution. Another known measure is to provide an exhaust trap in the exhaust passage to collect the particulates in the exhaust gas (as proposed in Unexamined Japanese Patent Publication 58-51235). This after-treatment method is considered the most practical and has been studied vigorously.
A particulate trap for collecting particulates contained in the exhaust gas has to meet the following requirements in view of the conditions of use:
1) Collecting Capacity
First, it has to be capable of collecting particulates with such high efficiency that the exhaust gas is cleaned sufficiently. The amount of particulates contained in the exhaust gas depends on the displacement of the diesel engine and the load applied. It is generally considered that such a trap has to be capable of collecting at least 60% of the particulates in the exhaust gas.
2) Pressure Drop
Secondly, such a trap must not unduly prevent the flow of exhaust gas. As the amount of the particulates collected by the trap increases, the pressure drop increases gradually. If such pressure drop is too high, an undesirable back pressure will act on the engine. It is considered necessary to keep such pressure drop below 30 KPa. For this purpose, it is necessary to use a particulate trap which is low not only in initial pressure drop but also keeps the pressure drop low even after it has collected particulates.
3) Regeneration
Thirdly, such a trap has to permit regeneration without requiring much energy. In other words, the trap has to have means for burning the collected particulates to regenerate it. One conceivable device for burning particulates is a light oil burner. However, considering the safety and ease of control, an electric heater is considered more promising. The trap has to be regenerated without consuming too much electric power because the capacity of a battery on a car is limited.
A wall-flow type honeycomb-shaped porous member made of cordierite ceramic is considered most practical as the filter element material in the particulate trap. However, with this type of filter, particulates tend to collect in a limited area. Further, due to low heat conductivity of cordierite ceramic, heat spots tend to develop when burning particulates. Thus, the filter may melt or develop cracks due to thermal stress. Such a filter is therefore not reliable enough.
Much attention is now directed to a metal trap and a ceramic fiber trap comprising candle-shaped ceramic fibers, because these traps never develop cracks and thus are sufficiently reliable.
Such traps have a structural problem in that their filtration area through which exhaust gas can pass is small compared with a cordierite ceramic filter. If the filter is designed so as to exhibit increased particulate collection efficiency, the particulates will be collected only on the surface of the filter, thus clogging it. The clogged filter will sharply increase the pressure drop. Thus, the life of such a filter is very short.
These filters have another disadvantage which is observed when burning particulates to regenerate them. Namely, whereas a cordierite ceramic filter can burn particulates with less power consumption because it heats itself up while burning the collected particulates and the heat thus generated is propagated to the particulates, metal traps and ceramic fiber traps cannot collect as much particulates as the cordierite ceramic filter and thus they are not heated to a sufficient degree. This means that the particulates have to be burned practically solely by the heat produced by the electric heater. Thus, the electric heater consumes much electric power.